There appears to be a transport barrier for oxygen between the red blood cell hemoglobin and the myocyte: intracellular oxygen tensions are lower than those in the venous outflow, and indicator dilution curves for 18O labeled oxygen give indication of barrier limitation. In the proposed study data obtained with both 18O and 15O will be examined with respect to the question of the location of the barrier or the relative permeability-surface area products of the succession of barriers lying between hemoglobin and mitochondrial enzymes. The 15O oxygen can still be measured after transformation to 15O water, while 18O water is indistinguishable from background. The combining of information from the two labels maximizes the possibility of a correct and complete analysis.